LED element and method of manufacturing the same

ABSTRACT

An LED element capable of further improving the light extraction efficiency and its manufacturing method are provided. 
     An LED element comprises a semiconductor lamination part that includes a light-emitting layer, a diffractive surface on which light emitted from the light-emitting layer is incident and on which projection parts are formed with a period larger than an optical wavelength of the light and smaller than a coherence length of the light and which reflects the incident light in a plurality of modes according to a Bragg diffraction condition and transmits the incident light in a plurality of modes according to the Bragg diffraction condition, and a reflecting surface that reflects light refracted by the diffractive surface so that the reflected light is incident on the diffractive surface again, wherein the semiconductor lamination part is formed on the diffractive surface without any void around the projection parts and a proportion of a flat part in the diffractive surface is 40% or more in a plan view thereof.

TECHNICAL FIELD

The present invention relates to an LED element and its manufacturingmethod.

BACKGROUND ART

An LED element provided with a group III nitride semiconductor that isformed on the front surface of a sapphire substrate and that includes alight-emitting layer, a diffraction surface that is provided on thefront surface side of the sapphire substrate, that allows incidence oflight emitted from the light-emitting layer, and that has depressionparts or projection parts whose period is grater than an opticalwavelength of the light and is smaller than coherent length of thelight, and an Al reflection layer that is formed on the back surfaceside of the substrate, that causes the light diffracted at thediffraction surface to reflect and to be incident on the diffractionsurface again is known (refer to Patent Literature 1). With this LEDelement, light transmitted by diffraction effect is incident on thediffraction surface again, and transmitted through the diffractionsurface by using the diffraction effect again, so that the light can beextracted to the outside of the element in a plurality of modes.

CITATION LIST Patent Literature

-   Patent Literature 1: WO2011/027679A1

SUMMARY OF INVENTION Technical Problem

The present inventors have pursued further improvement in lightextraction efficiency.

The present invention is made in view of the above-describedcircumstances, and its object is to provide an LED element capable offurther improving the light extraction efficiency, and its manufacturingmethod.

Solution to Problem

In order to achieve the above-described object, the present inventionprovides an LED element including: a semiconductor lamination part thatincludes a light-emitting layer; a diffractive surface on which lightemitted from the light-emitting layer is incident and on whichprojection parts are formed with a period larger than an opticalwavelength of the light and smaller than a coherence length of the lightand which reflects the incident light in a plurality of modes accordingto a Bragg diffraction condition and transmits the incident light in aplurality of modes according to the Bragg diffraction condition; and areflecting surface that reflects light refracted by the diffractivesurface so that the reflected light is incident on the diffractivesurface again, wherein the semiconductor lamination part is formed onthe diffractive surface without any void around the projection parts,and a proportion of a flat part in the diffractive surface is 40% ormore in a plan view thereof.

Moreover, the present invention provides a method of manufacturing theLED element, the method including: a mask layer forming step of forminga mask layer on a front surface of a sapphire substrate; a resist filmforming step of forming a resist film on the mask layer; a patternforming step of forming a predetermined pattern on the resist film; amask layer etching step of etching the mask layer using the resist filmas a mask; a substrate etching step of etching the sapphire substrateusing the etched mask layer as a mask to form the projection parts; anda semiconductor forming step of forming the semiconductor laminationpart on the etched front surface of the sapphire substrate.

Further, the present invention provides an LED element including: asapphire substrate; and a semiconductor lamination part including alight-emitting layer that is formed on a front surface of the sapphiresubstrate so as to emit blue light, wherein the front surface of thesapphire substrate has a plurality of depression parts or projectionparts disposed at the intersections of virtual triangular or rectangularlattices in a plan view thereof, and the triangles or rectangles thatform the virtual triangular or rectangular lattice do not have a regularpolygonal shape and the length of each side is larger than twice theoptical wavelength of the blue light and smaller than the coherencelength.

Further, the present invention provides an LED element including: asubstrate; a semiconductor lamination part that includes alight-emitting layer formed on a front surface of the substrate; areflecting portion formed on a back surface of the substrate; and anelectrode formed on the semiconductor lamination part, wherein theelectrode includes a diffusion electrode layer formed on thesemiconductor lamination part and a moth-eye layer which is formed onthe diffusion electrode layer and of which the front surface forms thetransmissive moth-eye surface having depression parts or projectionparts formed with a period smaller than twice the optical wavelength ofthe light emitted from the light-emitting layer, and the moth-eye layeris formed of a material which has a smaller extinction coefficient thana material that forms the diffusion electrode layer with respect to thelight emitted from the light-emitting layer and has approximately thesame index of refraction as the material that forms the diffusionelectrode layer.

Advantageous Effects of Invention

With the LED element according to the present invention, it is possibleto further improve the light extraction efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic sectional view of an LED element according to afirst embodiment of the present invention;

FIG. 2 are explanatory views illustrating diffraction effect of light atinterfaces having different indices of refraction, in which (a)illustrates the state where reflection is made at the interface, and (b)illustrates the state where transmission is made through the interface;

FIG. 3 is a graph illustrating the relationship between the angle ofincident light being incident on the interface from the side of asemiconductor layer and the angle of transmission at the interface bydiffraction effect, at the interface between a group III nitridesemiconductor layer and a sapphire substrate, when a period ofdepression parts or projection parts is set as 500 nm;

FIG. 4 is a graph illustrating the relationship between the angle ofincident light being incident on the interface from the side of thesemiconductor layer and the angle of reflection at the interface by thediffraction effect, at the interface between the group III nitridesemiconductor layer and the sapphire substrate, when the period of thedepression parts or the projection parts is set as 500 nm;

FIG. 5 is an explanatory view illustrating the traveling direction oflight in the element;

FIG. 6 is a partially enlarged schematic sectional view of the LEDelement;

FIG. 7 illustrate the sapphire substrate, in which (a) is a schematicperspective view, (b) is a schematic explanatory view taken along theA-A line, and (c) is a schematic enlarged explanatory view

FIG. 8 is a schematic plan view illustrating an arrangement state ofprojection parts, in which (a) illustrates a state where a virtualtriangular lattice has a regular triangular shape and (b) illustrates astate where a virtual triangular lattice has an isosceles triangularshape;

FIG. 9 is a graph illustrating the relation between the length of oneside and the light extraction efficiency when a virtual triangularlattice or a rectangular lattice has a regular polygonal shape;

FIG. 10 is a graph illustrating the relation between the length ofequilateral sides and the light extraction efficiency when a virtualtriangular lattice has an isosceles triangular shape;

FIG. 11 is a schematic explanatory view of a plasma etching apparatus;

FIG. 12 is a flowchart illustrating an etching method of the sapphiresubstrate;

FIG. 13A illustrate processes of the etching method of the sapphiresubstrate and a mask layer, in which (a) illustrates the sapphiresubstrate before processing, (b) illustrates the state where the masklayer is formed on the sapphire, (c) illustrates the state where aresist film is formed on the mask layer, (d) illustrates the state wherea mold is brought into contact with the resist film, and (e) illustratesthe state where a pattern is formed on the resist film;

FIG. 13B illustrate processes of the etching method of the sapphiresubstrate and the mask layer, in which (f) illustrates the state where aresidual film of the resist film is removed, (g) illustrates the statewhere the resist film is altered, (h) illustrates the state where themask layer is etched by using the resist film as a mask, and (i)illustrates the state where the sapphire substrate is etched by usingthe mask layer as a mask;

FIG. 13C illustrate processes of the etching method of the sapphiresubstrate and the mask layer, in which (j) illustrates the state wherethe sapphire substrate is etched further by using the mask layer as amask, (k) illustrates the state where the remaining mask layer isremoved from the sapphire substrate, and (l) illustrates the state wherethe sapphire substrate is subjected to wet-etching;

FIG. 14 is a graph illustrating the relation between the diameter of abase end of a projection part and the height of a projection part whenthe thickness of a Ni layer was changed;

FIG. 15 is a table illustrating the period of projection parts, theproportion of a C-surface area, and whether dislocation density is equalto or smaller than a predetermined value;

FIG. 16 is a graph illustrating reflectivity of a reflection partaccording to an example 1;

FIG. 17 is a graph illustrating the reflectivity of a reflection partaccording to an example 2;

FIG. 18 is a schematic sectional view of an LED element according to asecond embodiment of the present invention;

FIG. 19 is a partially enlarged schematic sectional view of the LEDelement;

FIG. 20 is a graph illustrating the reflectivity of a reflection partaccording to an example 3;

FIG. 21 is a graph illustrating the reflectivity of a reflection partaccording to an example 4;

FIG. 22 is a schematic sectional view of an LED element according to asecond embodiment of the present invention;

FIG. 23 is an explanatory view illustrating the traveling direction oflight inside the element;

FIG. 24 is an explanatory view for describing the processes ofprocessing a moth-eye layer, in which (a) illustrates a state where afirst mask layer is formed on a transmissive moth-eye surface, (b)illustrates a state where a resist layer is formed on the first masklayer, (c) illustrates a state where the resist layer is selectivelyirradiated with an electron beam, (d) illustrates a state where theresist layer is developed and removed, and (e) illustrates a state wherea second mask layer is formed; and

FIG. 25 is an explanatory view for describing the processes ofprocessing the moth-eye layer, in which (a) illustrates a state wherethe resist layer is completely removed, (b) illustrates a state wherethe first mask layer is etched using the second mask layer as a mask,(c) illustrates a state where the second mask layer is removed, (d)illustrates a state where the transmissive moth-eye surface is etchedusing the first mask layer as a mask, and (e) illustrates a state wherethe first mask layer is removed.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a schematic sectional view of an LED element according to afirst embodiment of the present invention.

In an LED element 1, as illustrated in FIG. 1, a semiconductorlamination part 19, formed by a group III nitride semiconductor layer,is formed on the front surface of a sapphire substrate 2. This LEDelement 1 is a flip-chip type, and light is mainly extracted from theback surface side of the sapphire substrate 2. The semiconductorlamination part 19 has a buffer layer 10, an n-type GaN layer 12, alight-emitting layer 14, an electron blocking layer 16, and a p-type GaNlayer 18 in this order from the sapphire substrate 2 side. A p-sideelectrode 27 is formed on the p-type GaN layer 18, and an n-sideelectrode 28 is formed on the n-type GaN layer 12.

As illustrated in FIG. 1, the buffer layer 10 is formed on the frontsurface of the sapphire substrate 2 and is formed by MN. According tothis embodiment, the buffer layer 10 is formed by an MOCVD (MetalOrganic Chemical Vapor Deposition) method, but may be formed by asputtering method. The n-type GaN layer 12, as a first conductivity typelayer, is formed on the buffer layer 10 and is formed by n-GaN. Thelight-emitting layer 14 is formed on the n-type GaN layer 12, formed byGaInN/GaN, and emits blue light by electron and hole injection. Here,the blue light means light whose peak wavelength is 430 nm or more and480 nm or less, for example. According to this embodiment, the peakwavelength of light emitted from the light-emitting layer 14 is 450 nm.

The electron blocking layer 16 is formed on the light-emitting layer 14,and is formed by p-AlGaN. The p-type GaN layer 18, as a secondconductivity type layer, is formed on the electron blocking layer 16,and is formed by p-GaN. The n-type GaN layer 12 to the p-type GaN layer18 are formed by epitaxial growth of the group III nitridesemiconductor, and projection parts 2 c are periodically formed on thefront surface of the sapphire substrate 2. However, at the beginning ofgrowth of the group III nitride semiconductor, planarization by lateralgrowth is made. Incidentally, the semiconductor layer may be constitutedfreely as long as it includes at least the first conductivity typelayer, an active layer, and the second conductivity type layer, and itemits light from the active layer by recombination of the electron andthe hole when a voltage is applied to the first conductivity type layerand the second conductivity type layer.

The front surface of the sapphire substrate 2 forms a verticalizingmoth-eye surface 2 a, and the back surface of the sapphire substrate 2forms a transmissive moth-eye surface 2 g. A flat part 2 b and aplurality of projection parts 2 c that are periodically formed on theflat part 2 b are formed on the front surface of the sapphire substrate2. In the present embodiment, the semiconductor lamination part 19 isformed around each projection part 2 c without any void. Examples of theshape of each projection part 2 c include a pyramidal shape such as aconical shape or a polygonal pyramidal shape and a truncated pyramidalshape obtained by cutting the top of a pyramid such as a truncatedconical shape or a truncated polygonal pyramidal shape. Each projectionpart 2 c is designed to diffract light emitted from the light-emittinglayer 14. In the present embodiment, the light verticalizing effect canbe obtained by the projection parts 2 c disposed periodically. Here, thelight verticalizing effect means an effect by which light intensitydistribution after the light is reflected from and transmitted throughthe verticalizing moth-eye surface is inclined closer to the directionvertical to the interface between the sapphire substrate 2 and thesemiconductor lamination part 19 than before the light is incident onthe verticalizing moth-eye surface.

In addition, on the back surface of the sapphire substrate 2, a flatpart 2 h and a plurality of projection parts 2 i that are periodicallyformed on the flat part 2 h are formed. The shape of each projectionpart 2 i may be a pyramid shape such as a cone, a polygonal pyramid orthe like, or may be a truncated pyramid shape, as a pyramid whose upperportion is cut off, such as a truncated cone, a truncated polygonalpyramid or the like. A period of the projection parts 2 i on thetransmission moth-eye surface is smaller than a period of the projectionparts 2 c on the verticalized moth-eye surface. According to thisembodiment, the respective projection parts 2 i, arranged periodically,suppress Fresnel reflection at the interface with the outside.

FIG. 2 are explanatory views illustrating diffraction effect of light atinterfaces having different indices of refraction, in which (a)illustrates the state where reflection is made on the interface, and (b)illustrates the state where transmission is made through the interface.

Here, from the Bragg diffraction condition, the condition to besatisfied by the angle of reflection θ_(rd) with respect to the angle ofincident θ_(in) at the time when light is reflected on the interface isas follows.d×n1×(sin θ_(in)−sin θ_(ref))=m×λ  (1)Wherein n1 is an index of refraction of a medium on the incident side,λ, is a wavelength of incident light, and m is an integer. When light isincident on the sapphire substrate 2 from the semiconductor laminationpart 19, n1 is the index of refraction of the group III nitridesemiconductor. As illustrated in FIG. 2(a), light being incident on theinterface is reflected at the angle of reflection θ_(ref) that satisfiesthe above-described expression (1).

Meanwhile, from the Bragg diffraction condition, the condition to besatisfied by the angle of transmission θ_(out) with respect to the angleof incident θ_(in) at the time when light is transmitted through theinterface is as follows.d×(n1×sin θ_(in) −n2×sin θ_(out))=m′×λ  (2)Wherein n2 is an index of refraction of a medium on the output side, andm′ is an integer. When, for example, light is incident on the sapphiresubstrate 2 from the semiconductor lamination part 19, n2 is the indexof refraction of sapphire. As illustrated in FIG. 2(b), the light beingincident on the interface is transmitted at the angle of transmissionθ_(out) satisfying the above-described expression (2).

For the existence of the angle of reflection θ_(ref) and the angle oftransmission θ_(out) satisfying the diffraction conditions of theabove-described expressions (1) and (2), the period on the front surfaceof the sapphire substrate 2 needs to be greater than (λ/n1) and (λ/n2)as optical wavelengths in the element. Therefore, the period on thefront surface of the sapphire substrate 2 is set to be greater than(λ/n1) and (λ/n2) so that diffraction light exists.

FIG. 3 is a graph illustrating the relationship between the angle ofincident light being incident on the interface from the semiconductorlayer side and the angle of transmission at the interface by thediffraction effect, at the interface between the group III nitridesemiconductor layer and the sapphire substrate, when a period of thedepression parts or the projection parts is set as 500 nm. In addition,FIG. 4 is a graph illustrating the relationship between the angle ofincident light being incident on the interface from the semiconductorlayer side and the angle of reflection at the interface by thediffraction effect, at the interface between the group III nitridesemiconductor layer and the sapphire substrate, when the period of thedepression parts or the projection parts is set as 500 nm.

As with the general flat surfaces, light being incident on theverticalized moth-eye surface 2 a has the critical angle of totalreflection. The critical angle at the interface between the GaN-basedsemiconductor layer and the sapphire substrate 2 is 45.9°. In the regionexceeding the critical angle, as illustrated in FIG. 3, transmission indiffraction modes of m′=1, 2, 3, and 4, satisfying the diffractioncondition of the above-described expression (2), is possible. Inaddition, in the region exceeding the critical angle, as illustrated inFIG. 4, reflection in diffraction modes of m=1, 2, 3, and 4, satisfyingthe diffraction condition of the above-described expression (1), ispossible. When the critical angle is 45.9°, light output exceeding thecritical angle is about 70%, and light output not exceeding the criticalangle is about 30%. Namely, extraction of light in the region exceedingthe critical angle greatly contributes to improvement of lightextraction efficiency of the LED element 1.

In the region where the angle of transmission θ_(out) is smaller thanthe angle of incident θ_(in), light that transmits through theverticalized moth-eye surface 2 a changes its angle toward the verticalwith respect to the interface between the sapphire substrate 2 and thegroup III nitride semiconductor layer. This region is hatched in FIG. 3.As illustrated in FIG. 3, in the region exceeding the critical angle,light that transmits through the verticalized moth-eye surface 2 a andthat is in the diffraction modes of m′=1, 2, and 3 changes its angletoward the vertical in all angle regions. Although the light in thediffraction mode of m′=4 does not change its angle toward the verticalin a part of the angle regions, it has not so much influence asintensity of light having a greater diffraction order is relativelysmall, and substantially, the light also changes its angle toward thevertical in this part of the angle regions. Namely, the intensitydistribution of the light transmitting through and extracted from theverticalized moth-eye surface 2 a on the sapphire substrate 2 side isinclined to the direction closer to the vertical with respect to theinterface between the semiconductor lamination part 19 and the sapphiresubstrate 2, as compared with the intensity distribution of the lightbeing incident on the verticalized moth-eye surface 2 a on thesemiconductor lamination part 19 side.

In the region where the angle of reflection θ_(ref) is smaller than theangle of incident θ_(in), light that is reflected on the verticalizedmoth-eye surface 2 a changes its angle toward the vertical with respectto the interface between the sapphire substrate 2 and the group IIInitride semiconductor layer. This region is hatched in FIG. 4. Asillustrated in FIG. 4, in the region exceeding the critical angle, lightthat is reflected on the verticalized moth-eye surface 2 a and that isin the diffraction modes of m=1, 2, and 3 changes its angle toward thevertical in all angle regions. Although the light in the diffractionmode of m=4 does not change its angle toward the vertical in a part ofthe angle regions, it has not so much influence as intensity of lighthaving a greater diffraction order is relatively small, andsubstantially, the light also changes its angle toward the vertical inthis part of the angle regions. Namely, the intensity distribution ofthe light that is extracted, by reflection, from the verticalizedmoth-eye surface 2 a on the semiconductor lamination part 19 side isinclined to the direction closer to the vertical with respect to theinterface between the semiconductor lamination part 19 and the sapphiresubstrate 2, as compared with the intensity distribution of the lightbeing incident on the verticalized moth-eye surface 2 a on thesemiconductor lamination part 19 side.

FIG. 5 is an explanatory view illustrating the traveling direction oflight in the element.

As illustrated in FIG. 5, light being incident on the sapphire substrate2 by exceeding the critical angle, among light emitted from thelight-emitting layer 14, is transmitted through and reflected on the theverticalized moth-eye surface 2 a toward the direction closer to thevertical, as compared with the direction when it is incident on theverticalized moth-eye surface 2 a. Namely, light that transmits throughthe verticalized moth-eye surface 2 a is incident on the transmissionmoth-eye surface 2 g by changing its angle toward the vertical. Further,light that is reflected on the verticalized moth-eye surface 2 a, whoseangle is changed toward the vertical, is reflected on the p-sideelectrode 27 and the n-side electrode 28, and thereafter, is incident onthe verticalized moth-eye surface 2 a again. The angle of incident atthis time is closer to the vertical than the angle of incident. As aresult of this, light being incident on the transmission moth-eyesurface 2 g can be directed toward the vertical.

FIG. 6 is a partially enlarged schematic sectional view of the LEDelement.

As illustrated in FIG. 6, the p-side electrode 27 includes a diffusionelectrode 21 that is formed on the p-type GaN layer 18, a dielectricmultilayer film 22 that is formed on the predetermined region on thediffusion electrode 21, and a metal electrode 23 that is formed on thedielectric multilayer film 22. The diffusion electrode 21 is formedentirely on the p-type GaN layer 18, and is formed by a transparentmaterial such as ITO (Indium Tin Oxide), for example. The dielectricmultilayer film 22 is formed by repeating a plurality of pairs of afirst material 22 a and a second material 22 b, having different indicesof refraction. For example, the dielectric multilayer film 22 may havefive pairs of the first material 22 a of ZrO₂ (index of refraction:2.18) and the second material 22 b of SiO₂ (index of refraction: 1.46).It should be noted that materials other than ZrO₂ and SiO₂ may be usedto form the dielectric multilayer film 22, and MN (index of refraction:2.18), Nb₂O₃ (index of refraction: 2.4), Ta₂O₃ (index of refraction:2.35) or the like may be used, for example. The metal electrode 23covers the dielectric multilayer film 22, and is formed by a metalmaterial such as Al, for example. The metal electrode 23 is electricallyconnected to the diffusion electrode 21 through a via hole 22 c formedin the dielectric multilayer film 22.

As illustrated in FIG. 6, the n-side electrode 28 is formed on then-type GaN layer 12 exposed after etching the p-type GaN layer 18 to then-type GaN layer 12. The n-side electrode 28 includes a diffusionelectrode 24 that is formed on the n-type GaN layer 12, a dielectricmultilayer film 25 that is formed on the predetermined region on thediffusion electrode 24, and a metal electrode 26 that is formed on thedielectric multilayer film 25. The diffusion electrode 24 is formedentirely on the n-type GaN layer 12, and is formed by a transparentmaterial such as ITO (Indium Tin Oxide), for example. The dielectricmultilayer film 25 is formed by repeating a plurality of pairs of afirst material 25 a and a second material 25 b, having different indicesof refraction. For example, the dielectric multilayer film 25 may havefive pairs of the first material 25 a of ZrO₂ (index of refraction:2.18) and the second material 25 b of SiO₂ (index of refraction: 1.46).It should be noted that materials other than ZrO₂ and SiO₂ may be usedto form the dielectric multilayer film 25, and MN (index of refraction:2.18), Nb₂O₃ (index of refraction: 2.4), Ta₂O₃ (index of refraction:2.35) or the like may be used, for example. The metal electrode 26covers the dielectric multilayer film 25, and is formed by a metalmaterial such as Al, for example. The metal electrode 26 is electricallyconnected to the diffusion electrode 24 through a via hole 25 a formedin the dielectric multilayer film 25.

In this LED element 1, the p-side electrode 27 and the n-side electrode28 form a reflection part. Reflectivity of the p-side electrode 27 andthe n-side electrode 28 becomes higher as the angle comes closer to thevertical. Light that is reflected on the verticalized moth-eye surface 2a of the sapphire substrate 2 and changes its angle toward the verticalwith respect to the interface, as well as light emitted from thelight-emitting layer 14 and being incident thereon directly, is incidenton the reflection part. Namely, the intensity distribution of lightbeing incident on the reflection part is inclined to the directioncloser to the vertical, as compared with the case where the frontsurface of the sapphire substrate 2 forms the flat surface.

Next, the sapphire substrate 2 will be described in detail withreference to FIG. 7. FIG. 7 illustrate the sapphire substrate, in which(a) is a schematic perspective view, (b) is a schematic explanatory viewtaken along with the A-A line, and (c) is a schematic enlargedexplanatory view.

As illustrated in FIG. 7(a), the verticalizing moth-eye surfaces 2 a areformed so as to be aligned at the intersections of virtual triangularlattices with a predetermined period so that the centers of therespective projection parts 2 c are positioned at the vertices ofregular triangles in a plan view thereof. The projection parts 2 c maybe disposed so that the centers of the projection parts 2 c arepositioned at the vertices of isosceles triangles. The period of therespective projection parts 2 c is larger than an optical wavelength ofthe light emitted from the light-emitting layer 14 and is smaller thanthe coherence length of the light. It should be noted that the periodmeans the distance between the peak height positions of the adjacentprojection parts 2 c. Further, the optical wavelength means a valueobtained by dividing an actual wavelength by the index of refraction.Further, the coherence length corresponds to the distance in whichperiodic vibrations of waves cancel each other due to a differencebetween individual wavelengths of a group of photons having apredetermined spectrum width so that coherence disappears. The coherencelength lc satisfies a relation of lc=(λ²/Δλ) in which the wavelength oflight is λ and a half-value width of the light is Δλ. When the period ofthe respective projection parts 2 c is one or more times larger than theoptical wavelength, the diffraction effect gradually and effectivelystarts acting on the incident light having the angle of the criticalangle or more. When the period is two or more times larger than theoptical wavelength of the light emitted from the light-emitting layer14, the number of transmission modes and reflection modes increasessufficiently, which is favorable. Moreover, it is preferable that theperiod of the respective projection parts 2 c is smaller than half thecoherence length of the light emitted from the light-emitting layer 14.

In the present embodiment, the length of one side of the regulartriangles that form the virtual triangular lattice is 460 nm. That is,the main period of the respective projection parts 2 c is 460 nm and 797nm. Moreover, the sapphire substrate 2 is configured so that theproportion of the flat parts 2 b is 40% or more in a plan view thereof.Since the wavelength of the light emitted from the light-emitting layer14 is 450 nm and the index of refraction of the group-III nitridesemiconductor layer is 2.4, the optical wavelength thereof is 187.5 nm.Further, since the half-value width of the light emitted from thelight-emitting layer 14 is 27 nm, the coherence length of the light is7500 nm. That is, the period of the verticalizing moth-eye surfaces 2 ais larger than twice the optical wavelength of the light-emitting layer14 and is equal to or smaller than half the coherence length.

FIG. 8 is a schematic plan view illustrating an arrangement state ofprojection parts, in which (a) illustrates a state where a virtualtriangular lattice has a regular triangular shape and (b) illustrates astate where a virtual triangular lattice has an isosceles triangularshape.

Here, as illustrated in FIG. 8(a), when the virtual triangular latticehas a regular triangular shape, six projection parts 2 c are present atan interval of 60° at the closest distance al from each projection part2 c. That is, as illustrated in FIG. 8(a), the closest projection parts2 c about a certain projection part 2 c are positioned in the directions0°, 60°, 120°, 180°, 240°, and 300°. Among these directions, thedirections 0°, 60°, and 120° are equivalent to the directions 180°,240°, and 300°, respectively.

Moreover, the projection parts 2 c at the next closest distance a2 arepositioned in the directions 30°, 90°, 150°, 210°, 270°, and 330°. Amongthese directions, the directions 30°, 90°, and 150° are equivalent tothe directions 210°, 270°, and 330°, respectively. That is, when thevirtual triangular lattice has a regular triangular shape, two types ofperiods of the distances a1 and a2 are mainly present.

On the other hand, as illustrated in FIG. 8(b), when the virtualtriangular lattice has an isosceles triangular shape and the base angleof the isosceles triangle is θ, the projection parts 2 c positioned atthe distance b1 of the equilateral sides from each projection part 2 care positioned in the directions θ, (180°−θ), (180°+θ), and (360°−θ).Among these directions, the directions θ and (180°−θ) are equivalent tothe directions (180°+θ) and (360°−θ), respectively.

Moreover, the projection parts 2 c positioned at the distance b2 of thebottom side from each projection part 2 c are positioned in thedirections 0° and 180°. These directions are equivalent directions.

Further, the projection parts 2 c positioned at the distance b3 of(2×b1×sin θ) from each projection part 2 c are positioned at thedirections 90° and 270°. These directions are equivalent directions.

Further, the projection parts 2 c positioned at the distance b4 of((3/2×b2)²+(b1×sin θ)²)^(1/2) from each projection part 2 c arepositioned in the directions Tan⁻¹(b3/3×b2), (180°−Tan⁻¹(b3/3×b2)),(180°+Tan⁻¹(b3/3×b2)), and (360°−Tan⁻¹(b3/3×b2)). Among thesedirections, the directions Tan⁻¹(b3/3×b2) and (180°−Tan⁻¹(b3/3×b2)) areequivalent to the directions (180°+Tan⁻¹(b3/3×b2)) and(360°−Tan⁻¹(b3/3×b2)), respectively.

That is, when the virtual triangular lattice has an isosceles triangularshape, four types of periods of the distances b1, b2, b3, and b4 aremainly present, and the number of diffraction modes which can be usedfor light extraction increases.

FIG. 9 is a graph illustrating the relation between the length of oneside and the light extraction efficiency when a virtual triangularlattice or a rectangular lattice has a regular polygonal shape.

As illustrated in FIG. 9, the relation between the length of one sideand the light extraction efficiency was calculated by simulation whenthe virtual triangular or rectangular lattice had a regular polygonalshape. Specifically, the wavelength of light was 450 nm, and thetransmittivity of light from a GaN-based semiconductor to a sapphiresubstrate at the interface between the GaN-based semiconductor and thesapphire substrate was calculated.

The results showed that relatively satisfactory transmittivity wasobtained for the virtual triangular lattice when the length of one sidewas 460 nm or smaller and between 550 nm and 800 nm. Moreover,relatively satisfactory transmittivity was obtained for the virtualrectangular lattice when the length of one side was 500 nm or smaller.

FIG. 10 is a graph illustrating the relation between the length ofequilateral sides and the light extraction efficiency when a virtualtriangular lattice has an isosceles triangular shape.

As illustrated in FIG. 10, the relation between the length ofequilateral sides and the light extraction efficiency was calculated bysimulation when the virtual triangular lattice had an isoscelestriangular shape. Specifically, the length of the bottom side was 600nm, the wavelength of light was 450 nm, and the transmittivity of lightfrom a GaN-based semiconductor to a sapphire substrate at the interfacebetween the GaN-based semiconductor and the sapphire substrate wascalculated.

The results showed that relatively satisfactory transmittivity wasobtained when the length of equilateral sides was 460 nm or smaller andbetween 550 nm and 800 nm similarly to the case where the virtualtriangular lattice had a regular triangular shape.

Moreover, when the length of the equilateral sides of an isoscelestriangle was the same as the length of one side of a regular triangle,higher transmittivity was obtained when the triangular lattice had anisosceles triangular shape than when the triangular lattice had aregular triangular shape. Specifically, the transmittivity increased by4% when the length of the equilateral sides and one side was 400 nm, 5%when the length was 460 nm, 1% when the length was 500 nm, and 1% whenthe length was 700 nm. Since the length of the bottom side of theisosceles triangle was 600 nm, when the length of the equilateral sideswas 600 nm, the same transmittivity as that of the regular triangularlattice was obtained. Moreover, when the length of the equilateral sideswas 800 nm, approximately the same transmittivity as that of the regulartriangular lattice whose side was 800 nm long was obtained.

From the above, it can be understood that higher light extractionefficiency is obtained for the virtual triangular lattice having theisosceles triangular shape than the virtual triangular lattice havingthe regular triangular shape. In this case, the length of theequilateral sides and the bottom side of the isosceles triangle ispreferably 460 nm or smaller or between 550 nm and 800 nm. It can bealso understood that the relation between the angle of incidence oflight and the transmittivity in the length region of 460 nm or smalleris different from that in the length region between 550 nm and 800 nm.That is, further preferably, one of the length of the equilateral sidesand the bottom side of the isosceles triangular is 460 nm or smaller andthe length of the other is between 550 nm and 800 nm.

According to this embodiment, as illustrated in FIG. 7(c), eachprojection part 2 c on the verticalized moth-eye surface 2 a includes aside surface 2 d that extends upward from the flat part 2 b, a bentportion 2 e that bends and extends from the upper end of the sidesurface 2 d toward the center side of the projection part 2 c, and aflat top surface 2 f that is formed continuously from the bent portion 2e. As will be described later, before the formation of the bent portion2 e, the projection part 2 c, on which a corner is formed at a portionassociating the side surface 2 d and the top surface 2 f, is wet-etchedand rounded, and thus the bent portion 2 e is formed. The wet-etchingmay be made until the flat top surface 2 f eliminates and the entireupper side of the projection part 2 c becomes the bent portion 2 e.Specifically, according to this embodiment, the diameter of the base endportion of each projection part 2 c is 380 nm, and its height is 350 nm.In the verticalized moth-eye surface 2 a of the sapphire substrate 2,the flat part 2 b is provided at the position where the projection parts2 c are not provided, thus facilitating the lateral growth of thesemiconductor

Moreover, the transmissive moth-eye surfaces 2 g on the back surface ofthe sapphire substrate 2 are formed so as to be aligned at theintersections of virtual triangular lattices with a predetermined periodso that the centers of the respective projection parts 2 i arepositioned at the vertices of regular triangles in a plan view thereof.The shortest period of the projection parts 2 i is smaller than twicethe optical wavelength of the light emitted from the light-emittinglayer 14. In the present embodiment, the length of one side of theregular triangles that form the virtual triangular lattice is 300 nm.That is, the shortest period of the projection parts 2 i is 300 nm.Since the wavelength of the light emitted from the light-emitting layer14 is 450 nm and the index of refraction of sapphire is 1.78, theoptical wavelength thereof is 252.8 nm. That is, the shortest period ofthe transmissive moth-eye surfaces 2 g is smaller than twice the opticalwavelength of the light-emitting layer 14. When the period of themoth-eye surfaces is equal to or smaller than twice the opticalwavelength, the Fresnel reflection at the interface can be suppressed.It is possible to sufficiently obtain the effect of suppressing theFresnel reflection when the shortest period of the projection parts 2 iis smaller than twice the optical wavelength. Moreover, it is possibleto obtain a larger effect of suppressing the Fresnel reflection when allperiods of the projection parts 2 i are smaller than twice the opticalwavelength. The effect of suppressing the Fresnel reflection increasesas the period of the transmissive moth-eye surface 2 g approaches onetimes from two times the optical wavelength. When the outside of thesapphire substrate 2 is resin or air, and when the period of thetransmissive moth-eye surfaces 2 g is equal to or smaller than 1.25times the optical wavelength, it is possible to obtain approximately thesame effect of suppressing the Fresnel reflection as when the period isone times or smaller than the optical wavelength.

Here, a method of manufacturing the sapphire substrate 2 for the LEDelement 1 will be explained with reference to FIG. 11 to FIG. 13C. FIG.11 is a schematic explanatory view of a plasma etching apparatus forprocessing a sapphire substrate.

As illustrated in FIG. 11, a plasma etching apparatus 91 is an inductivecoupling (ICP) type plasma etching apparatus and includes a flatplate-shaped substrate holding table 92 that holds the sapphiresubstrate 2, a container 93 that receives the substrate holding table92, a coil 94 that is provided above the container 93 with a quartzplate 96 interposed, and a power supply 95 that is connected to thesubstrate holding table 92. The coil 94 has a solid spiral coil andsupplies high-frequency power from the center of the coil and aterminating end at the outer periphery of the coil is grounded. Thesapphire substrate 2 to be etched is placed on the substrate holdingtable 92 directly or with a carrier tray interposed. A cooling mechanismfor cooling the sapphire substrate 2 is included in the substrateholding table 92 and is controlled by a cooling controller unit 97. Thecontainer 93 has a supply port through which various gases such as an O₂gas, an Ar gas, and the like can be supplied.

When the etching is made by this plasma etching apparatus 1, thesapphire substrate 2 is placed on the substrate holding table 92 andthen, air inside the container 93 is discharged to attain a decompressedstate. The predetermined processing gas is supplied into the container93, and gas pressure inside the container 93 is adjusted. Thereafter,high-output and high-frequency power is supplied to the coil 94 and thesubstrate holding table 92 for the predetermined period of time, andplasma 98 of a reaction gas is formed. This plasma 98 is used foretching the sapphire substrate 2.

Next, an etching method by using the plasma etching apparatus 91 will beexplained with reference to FIG. 12, FIG. 13A, FIG. 13B and FIG. 13C.

FIG. 12 is a flowchart illustrating the etching method. As illustratedin FIG. 12, the etching method according to this embodiment includes amask layer formation process S1, a resist film formation process S2, apattern formation process S3, a residual film removal process S4, aresist alteration process S5, a mask layer etching process S6, asapphire substrate etching process S7, a mask layer removal process S8,and a bent portion formation process S9.

FIG. 13A illustrate processes of the etching method of the sapphiresubstrate and the mask layer, in which (a) illustrates the sapphiresubstrate before processing, (b) illustrates the state where the masklayer is formed on the sapphire substrate, (c) illustrates the statewhere a resist film is formed on the mask layer, (d) illustrates thestate where a mold is brought into contact with the resist film, and (e)illustrates the state where a pattern is formed on the resist film.

FIG. 13B illustrate processes of the etching method of the sapphiresubstrate and the mask layer, in which (f) illustrates the state where aresidual film of the resist film is removed, (g) illustrates the statewhere the resist film is altered, (h) illustrates the state where themask layer is etched by using the resist film as a mask, and (i)illustrates the state where the sapphire substrate is etched by usingthe mask layer as a mask. It should be noted that the resist film afterthe alteration is filled in with black in the drawings.

FIG. 13C illustrate processes of the etching method of the sapphiresubstrate and the mask layer, in which (j) illustrates the state wherethe sapphire substrate is etched further by using the mask layer as amask, (k) illustrates the state where the remaining mask layer isremoved from the sapphire substrate, and (l) illustrates the state wherethe sapphire substrate is subjected to the wet-etching.

First, as illustrated in FIG. 13A(a), the sapphire substrate 2 beforeprocessing is provided. Prior to the etching, the sapphire substrate 2is cleaned by the predetermined cleaning liquid. According to thisembodiment, the sapphire substrate 2 is a substrate formed by sapphire.

Then, as illustrated in FIG. 13A(b), a mask layer 30 is formed on thesapphire substrate 2 (mask layer formation process: S1). According tothis embodiment, the mask layer 30 includes a SiO₂ layer 31 on thesapphire substrate 2, and a Ni layer 32 on the SiO₂ layer 31. Thethickness of each of the layers 31 and 112 may be freely set, but theSiO₂ layer may be set to have the thickness of 1 nm or more and 100 nmor less, and the Ni layer 32 may be set to have the thickness of 1 nm ormore and 200 nm or less, for example. Incidentally, the mask layer 30may have a single layer. The mask layer 30 is formed by the sputteringmethod, a vacuum deposition method, a CVD method, or the like.

Next, as illustrated in FIG. 13A(c), the resist film 40 is formed on themask layer 30 (resist film formation process: S2). According to thisembodiment, the resist film 40 is formed by thermoplastic resin, and isformed by a spin coating method to have the uniform thickness. Theresist film 40 is formed by, for example, epoxy-based resin, and itsthickness is 100 nm or more and 300 nm or less, for example.Incidentally, it is also possible to use photosetting resin as theresist film 40.

The resist film 40, together with the sapphire substrate 2, is heatedand softened and, as illustrated in FIG. 13A(d), the resist film 40 ispressed by a mold 50. A projection-and-depression structure 51 is formedon the contact surface of the mold 50, and the resist film 40 isdeformed along the projection-and-depression structure 51.

Thereafter, the resist film 40, while being pressed, is cooled andhardened, together with the sapphire substrate 2. The mold 50 is thenseparated from the resist film 40 and, as illustrated in FIG. 10A(e), aprojection-and-depression structure 41 is transferred to the resist film40 (pattern formation process: S3). Here, the period of theprojection-and-depression structure 41 is 1 μm or less. According tothis embodiment, the period of the projection-and-depression structure41 is 460 nm. Further, according to this embodiment, the diameter of aprojection part 43 of the projection-and-depression structure 41 is 100nm or more and 300 nm or less, and is 230 nm, for example. Furthermore,the height of the projection part 43 is 100 nm or more and 300 nm orless, and is 250 nm, for example. In this state, a residual film 42 isformed on a depression part of the resist film 40.

The sapphire substrate 2, on which the resist film 40 is formed asdescribed above, is mounted on the substrate holding table 92 of theplasma etching apparatus 1. Then, the residual film 42 is removed byplasma ashing, for example, and the mask layer 30, as the material to beprocessed, is exposed, as illustrated in FIG. 13B(f) (residual filmremoval process: S4). According to this embodiment, the O₂ gas is usedas the processing gas for the plasma ashing. At this time, theprojection part 43 of the resist film 40 is subjected to the influenceof the ashing, and a side surface 44 of the projection part 43 is tiltedby the predetermined angle, not being vertical to the front surface ofthe mask layer 30.

Then, as illustrated in FIG. 13B(g), the resist film 40 is exposed tothe plasma under an alteration condition, so as to alter the resist film40 and increase etch selectivity (resist alteration process: S5).According to this embodiment, the Ar gas is used as the processing gasfor altering the resist film 40. Further, with regard to the alterationcondition according to this embodiment, bias output of the power supply95 for guiding the plasma to the sapphire substrate 2 side is set to belower than that of a later-described etching condition.

Then, the resist film 40, having the high etch selectivity after beingexposed to the plasma under the etching condition, is used as a mask toetch the mask layer 30 as the material to be processed (mask layeretching process: S6). According to this embodiment, the Ar gas is usedas the processing gas for etching the resist film 40. Thereby, asillustrated in FIG. 13B(h), a pattern 33 is formed on the mask layer 30.

With regard to the alteration condition and the etching condition, it ispossible to change the processing gas, antenna output, the bias outputand the like as appropriate, but it is preferable to change the biasoutput by using the same processing gas, as in this embodiment.Specifically, with regard to the alteration condition, the Ar gas is setas the processing gas, the antenna output of the coil 94 is set as 350W, and the bias output of the power supply 95 is set as 50 W, as aresult of which the hardening of the resist film 40 is observed.Further, with regard to the etching condition, the Ar gas is set as theprocessing gas, the antenna output of the coil 94 is set as 350 W, andthe bias output of the power supply 95 is set as 100 W, as a result ofwhich the etching of the mask layer 30 is observed. It should be notedthat the hardening of the resist is possible when the antenna output islowered and a gas flow rate is reduced, as well as when the bias outputis lowered, with respect to the etching condition.

Next, as illustrated in FIG. 13B(i), the sapphire substrate 2 is etchedby using the mask layer 30 as a mask (sapphire substrate etchingprocess: S7). According to this embodiment, the etching is made whilethe resist film 40 remains on the mask layer 30. Further, plasma etchingis made by using a chlorine-based gas, such as a BCl₃ gas, as theprocessing gas.

When the etching progresses, as illustrated in FIG. 13C(j), theverticalized moth-eye surface 2 a is formed on the sapphire substrate 2.According to this embodiment, the height of theprojection-and-depression structure on the verticalized moth-eye surface2 a is 350 nm. Incidentally, the height of the projection-and-depressionstructure may be increased to be greater than 350 nm. When the height ofthe projection-and-depression structure is relatively small, such as 300nm, for example, the etching may be finished while the remaining resistfilm 40 exists, as illustrated in FIG. 13B(i).

According to this embodiment, side etching is facilitated by the SiO₂layer 31 of the mask layer 30, and the side surface 2 d of theprojection part 2 c on the verticalized moth-eye surface 2 a is tilted.Further, a tilt angle of the side surface 43 of the resist film 40 canalso control the state of the side etching. It should be noted that,when the mask layer 30 is made as a single layer of the Ni layer 32, theside surface 2 d of the projection part 2 c can be made almost verticalto the main surface.

Moreover, in the present embodiment, the size at the base end of theprojection part 2 c is controlled by the thickness of the Ni layer 32.The present inventors have found that the diameter at the base end ofthe projection part 2 c can be adjusted by controlling the thickness ofthe Ni layer 32 as a metal mask. FIG. 14 is a graph illustrating therelation between the diameter of a base end of a projection part and theheight of a projection part when the thickness of a Ni layer waschanged. In this test, data was obtained by changing the thickness ofthe Ni layer 32 and the height of the projection part 2 c while usingthe same mold 50. Specifically, three types of thicknesses of 50 nm, 75nm, and 100 nm were used for the Ni layer 32, and four types of heightsof 400 nm, 500 nm, 600 nm, and 700 nm were used for the projection part2 c. In some samples, the projection part 2 c after etching did not havea desired height. As illustrated in FIG. 14, it can be understood thatthe thicker the Ni layer 32, the larger the diameter at the base end ofthe projection part 2 c was obtained. In this way, it is possible tochange the diameter at the base end of the projection part 2 c withoutchanging the mold 50.

Thereafter, as illustrated in FIG. 13C(k), the predetermined strippingliquid is used to remove the mask layer 30 remaining on the sapphiresubstrate 2 (mask layer removal process: S8). According to thisembodiment, high-temperature nitric acid is used to remove the Ni layer32, and then, hydrofluoric acid is used to remove the SiO₂ layer 31.When the resist film 40 remains on the mask layer 30, it can be removedtogether with the Ni layer 32 by the high-temperature nitric acid.However, when the remaining amount of the resist film 40 is large, it ispreferable to remove the resist film 40 by O₂ ashing in advance.

Then, as illustrated in FIG. 13C(1), the corner on the projection partpart 2 c is removed by the wet-etching, so as to form the bent portion(bent portion formation process: S9). Although the etching solution canbe freely selected, it is possible to use the so-called “hot phosphoricacid” as phosphoric acid aqueous solution that is heated to about 170°C., for example. Incidentally, this bent portion formation process canbe omitted as appropriate. After the above-described processes, thesapphire substrate 2 having the projection-and-depression structure onits front surface is manufactured.

According to this etching method of the sapphire substrate 2, thealteration of the resist film 40 is made by exposing itself to theplasma, and thus the etching selectivity of the mask layer 30 and theresist film 40 can be improved. This makes it possible to facilitate theprocessing of the fine and deep pattern on the mask layer 30, and toform the mask layer 30, having the fine pattern, with enough thickness.

Further, the plasma etching apparatus 1 can alter the resist film 40 andetch the mask layer 30 in a continuous manner, without significantlyincreasing man-hour. According to this embodiment, the alteration of theresist film 40 and the etching of the mask layer 30 are made by changingthe bias output of the power supply 95, which makes it possible toincrease the selectivity of the resist film 40 with ease.

Furthermore, as the mask layer 30, having the enough thickness, is usedas the mask to etch the sapphire substrate 2, the processing of the fineand deep pattern on the sapphire substrate 2 is facilitated. Especially,according to the etching method of this embodiment, it is possible toform the projection-and-depression structure having the period of 1 μmor less and the depth of 300 nm or more on the sapphire substrate, whichhas been impossible with the conventional etching method that forms theresist film on the substrate on which the mask layer is formed and thatuses the resist film for etching the mask layer. Especially, the etchingmethod according to this embodiment is suitable for forming theprojection-and-depression structure having the period of 1 μm or lessand the depth of 500 nm or more.

The nano-scaled periodic projection-and-depression structure is referredto as the moth-eye. When sapphire is subjected to this processing of themoth-eye, the processing is possible only to the depth of about 200 nm,as sapphire is a material that is difficult to grind. In some cases,however, difference in level of about 200 nm is not enough for themoth-eye. It is possible to say that the etching method according tothis embodiment solves this new problem at the time when the sapphiresubstrate is subjected to the moth-eye processing.

It is needless to say that, although the mask layer 30 formed by SiO₂/Niis presented as the material to be processed, the mask layer 30 may be asingle layer of Ni or may be formed by other materials. What is requiredis to alter the resist and increase the etch selectivity of the masklayer 30 and the resist film 40.

In addition, the case of setting the alteration condition and theetching condition by changing the bias output of the plasma etchingapparatus 1 is presented, but the setting may be made by changing theantenna output, the gas flow rate, or the processing gas, for example.What is required for the alteration condition is that the resist alterswhen being exposed to the plasma so as to increase the etch selectivity.

In addition, the mask layer 30 including the Ni layer 32 is presented,but it is needless to say that the present invention can be applied tothe etching of other materials. The etching method of the sapphiresubstrate according to this embodiment can be applied to a substrate ofSiC, Si, GaAs, GaN, InP, ZnO or the like.

The semiconductor lamination part 19 formed by the group III nitridesemiconductor is formed by the epitaxial growth on thus-manufacturedverticalized moth-eye surface 2 a of the sapphire substrate 2 by usingthe lateral growth (semiconductor formation process), on which thep-side electrode 27 and the n-side electrode 28 are formed (electrodeformation process). Thereafter, the projection parts 2 i are formed onthe back surface of the sapphire substrate 2 according to the sameprocesses as those used for the verticalized moth-eye surface 2 a on thefront surface, which is diced and divided into a plurality of the LEDelements 1. Thus, the LED element 1 is manufactured.

Here, the present inventors have examined whether dislocation densitywas equal to or smaller than a predetermined value when the proportionof the flat parts 2 b of the sapphire substrate 2 was changed so thatthe semiconductor lamination part 19 had a predetermined thickness.Specifically, it was examined whether the dislocation density was equalto or smaller than 2×10⁸/cm² when the flat part 2 b was used as aC-surface and the period and the like of the projections 2 c werechanged so that the semiconductor lamination part 19 had a thickness of2.5 μm. Samples which were formed so as to be aligned at theintersections of triangular lattices so that the projection parts 2 cwere positioned at the vertices of regular triangles were examined. Theexamination results are illustrated in FIG. 15. FIG. 15 is a tableillustrating the period of the projection parts 2 c, the proportion ofthe C-surface area, and whether dislocation density is equal to orsmaller than a predetermined value. The period illustrated in FIG. 15 isa period corresponding to the length of one side of a regular triangle.

As illustrated in FIG. 15, it can be understood that the dislocationdensity is 2×10⁸/cm² or smaller when the period of the projection parts2 c is 600 nm or more. It can be also understood that the dislocationdensity is 2×10⁸/cm² or smaller when the proportion of the flat part 2 bis 41% or more even if the period is 460 nm. That is, satisfactorycrystal quality is obtained even when the semiconductor lamination part19 is thin by setting the period of the projection parts 2 c to 600 nmor more or setting the proportion of the flat part 2 b to 41% or more.

In the LED element 1 having such a configuration, it is possible todecrease the thickness of the semiconductor lamination part 19 withoutdeteriorating the crystal quality of the light-emitting layer 14 bysetting the proportion of the flat part 2 b to 41% or more. Moreover, itis possible to improve the crystal quality of the light-emitting layer14 and to further improve the light extraction efficiency as long as thesemiconductor lamination part 19 is as thick as the conventionalsemiconductor lamination part.

Moreover, it is possible to control the diameter at the base end of theprojection part 2 c by controlling the thickness of the metal mask.Further, it is possible to manufacture different projection parts 2 cusing the same mold 50 and to share the mold 50 to reduce themanufacturing cost.

Moreover, since the verticalizing moth-eye surface 2 a is provided,therefore light being incident on the interface between the sapphiresubstrate 2 and the group III nitride semiconductor layer, by exceedingthe critical angle of total reflection, can be directed toward thevertical with respect to the interface. In addition, as the transmissionmoth-eye surface 2 g that suppresses the Fresnel reflection is provided,it is possible to smoothly extract light, whose angle is directed towardthe vertical, to the outside of the element, at the interface betweenthe sapphire substrate 2 and the outside of the element. Although thefront surface and the back surface of the sapphire substrate 2 are bothprocessed to have the projections and the depressions, both havedifferent functions of the verticalizing function and the Fresnelreflection suppressing function, and the light extraction efficiency canbe dramatically improved due to synergy between these functions.

Further, the distance of light, emitted from the light-emitting layer14, until reaching the back surface of the sapphire substrate 2, can bereduced substantially, and the absorption of light in the element can besuppressed. The LED element has such a problem that light is absorbed inthe element as light in the angle region exceeding the critical angle ofthe interface propagates laterally. However, light in the angle regionexceeding the critical angle is directed toward the vertical at theverticalized moth-eye surface 2 a, and the Fresnel reflection of thelight that is directed toward the vertical is suppressed at thetransmission moth-eye surface 2 g, and thus the light absorbed in theelement can be reduced drastically.

Moreover, light can be extracted using a large number of diffractionmodes when the front surface of the sapphire substrate 2 has a pluralityof projection parts 2 c disposed at the intersections of virtualtriangular lattices in a plan view thereof, and the triangles that formthe virtual triangular lattice do not have a regular polygonal shape. Inparticular, the light extraction efficiency can be improved when thelength of one side of triangles that form the virtual triangular latticeis twice or more than the optical wavelength of blue light and is 460 nmor smaller or between 550 nm and 800 nm. Moreover, when the trianglesthat form the virtual triangular lattice have an isosceles triangularshape, it is possible to increase the number of diffraction modes whileregularly disposing the projection parts 2 c. Further, light can beextracted using diffraction modes having different properties when thelength of equilateral sides of an isosceles triangle is twice or morethan the optical wavelength of blue light and is 400 nm or smaller andthe length of the bottom side is between 550 nm and 800 nm.

Here, the present inventors have found out that, by using thecombination of the dielectric multilayer films 22 and 25 and the metallayers 23 and 26 as the p-side electrode 27 and the n-side electrode 28,the light extraction efficiency of the LED element 1 increasessubstantially. Namely, when the dielectric multilayer films 22 and 25and the metal layers 23 and 26 are combined, the reflectivity increasesas the angle comes closer to the vertical with respect to the interface,which attains favorable reflection condition for light that is directedtoward the vertical with respect to the interface.

FIG. 16 is a graph illustrating the reflectivity of the reflection partaccording to an example 1. According to the example 1, five pairs ofZrO₂ and SiO₂ are combined to form the dielectric multilayer film onITO, and the Al layer is formed to overlap the dielectric multilayerfilm. As illustrated in FIG. 16, the reflectivity of 98% or more isrealized in the angle region where the angle of incident is from 0degree to 45 degrees. Further, the reflectivity of 90% or more isrealized in the angle region where the angle of incident is from 0degree to 75 degrees. Thus, the combination of the dielectric multilayerfilm and the metal layer is favorable as the reflection condition forlight that is directed toward the vertical with respect to theinterface.

FIG. 17 is a graph illustrating the reflectivity of a reflection partaccording to an example 2. According to the example 2, only the Al layeris formed on ITO. As illustrated in FIG. 17, the reflectivity shows 84%almost constantly, irrespective of the angle of incident. Thus, thereflection part may be a single layer of metal, such as the Al layer.

FIG. 18 is a schematic sectional view of an LED element according to asecond embodiment of the present invention.

In this LED element 101, as illustrated in FIG. 18, a semiconductorlamination part 119 formed by a group III nitride semiconductor layer isformed on the front surface of a sapphire substrate 102. This LEDelement 101 is a face-up type, and light is mainly extracted from theside opposite to the sapphire substrate 102. The semiconductorlamination part 119 has a buffer layer 110, an n-type GaN layer 112, alight-emitting layer 114, an electron blocking layer 116, and a p-typeGaN layer 118 in this order from the sapphire substrate 102 side. Ap-side electrode 127 is formed on the p-type GaN layer 118, and ann-side electrode 128 is formed on the n-type GaN layer 112.

As illustrated in FIG. 18, the buffer layer 110 is formed on the frontsurface of the sapphire substrate 102, and is formed by MN. The n-typeGaN layer 112 is formed on the buffer layer 110, and is formed by n-GaN.The light-emitting layer 114 is formed on the n-type GaN layer 112, andis formed by GaInN/GaN. According to this embodiment, a peak wavelengthof light emitted from the light-emitting layer 114 is 450 nm.

The electron blocking layer 116 is formed on the light-emitting layer114, and is formed by p-AlGaN. The p-type GaN layer 118 is formed on theelectron blocking layer 116, and is formed by p-GaN. The n-type GaNlayer 112 to the p-type GaN layer 118 are formed by epitaxial growth ofthe group III nitride semiconductor, and projection parts 102 c areperiodically formed on the front surface of the sapphire substrate 102.However, at the beginning of growth of the group III nitridesemiconductor, planarization by lateral growth is made. Incidentally,the semiconductor layer may be constituted freely as long as it includesat least a first conductivity type layer, an active layer, and a secondconductivity type layer, and it emits light from the active layer byrecombination of an electron and a hole when a voltage is applied to thefirst conductivity type layer and the second conductivity type layer.

In the present embodiment, the front surface of the sapphire substrate102 forms a verticalizing moth-eye surface 102 a, and the p-sideelectrode 127 forms a transmissive moth-eye surface 127 g. A flat part102 b and the plurality of projections 102 c that are periodicallyformed on the flat part 102 b are formed on the front surface of thesapphire substrate 102. Moreover, the sapphire substrate 102 isconfigured such that the proportion of the flat part 102 b is 40% ormore in a plan view thereof. Examples of the shape of each projection102 c include a pyramidal shape such as a conical shape or a polygonalpyramidal shape and a truncated pyramidal shape obtained by cutting thetop of a pyramid such as a truncated conical shape or a truncatedpolygonal pyramidal shape. Each projection 102 c is designed to diffractlight emitted from the light-emitting layer 114. In the presentembodiment, the light verticalizing effect can be obtained by theprojections 102 c disposed periodically.

The p-side electrode 127 includes a diffusion electrode 121 that isformed on the p-type GaN layer 118, and a pad electrode 122 that isformed on a part of the diffusion electrode 121. The diffusion electrode121 is formed entirely on the p-type GaN layer 118, and is formed by atransparent material such as ITO (Indium Tin Oxide), for example. Thepad electrode 122 is formed by a metal material such as Al, for example.On the front surface of the diffusion electrode 121, a flat part 127 hand a plurality of projection parts 127 i that are periodically formedon the flat part 127 h are formed. The shape of each projection part 127i may be a pyramid shape such as a cone, a polygonal pyramid or thelike, or may be a truncated pyramid shape, as a pyramid whose upperportion is cut off, such as a truncated cone, a truncated polygonalpyramid or the like. A period of the projection parts 127 i on thetransmission moth-eye surface is less than twice an optical wavelengthof the light-emitting layer 114. According to this embodiment, therespective projection parts 127 i arranged periodically suppress theFresnel reflection at the interface with the outside.

The n-side electrode 128 is formed on the n-type GaN layer 112 exposedafter etching the p-type GaN layer 118 to the n-type GaN layer 112. Then-side electrode 128 is formed on the n-type GaN layer 112, and isformed by a metal material such as Al, for example.

FIG. 19 is a partially enlarged schematic sectional view of the LEDelement.

As illustrated in FIG. 19, a dielectric multilayer film 124 is formed onthe back surface of the sapphire substrate 102. The dielectricmultilayer film 124 is formed by repeatedly stacking a plurality ofpairs of a first material 124 a and a second material 124 b, havingdifferent indices of refraction. The dielectric multilayer film 124 iscovered by an Al layer 126 which is a metal layer. In thislight-emitting element 101, the dielectric multilayer film 124 and theAl layer 126 form a reflecting portion, and light emitted from thelight-emitting layer 114 and transmitted through the verticalizingmoth-eye surface 102 a by the diffraction effect is reflected from thereflecting portion. The light transmitted by the diffraction effect isincident on the diffractive surface 102 a again and is transmittedthrough the diffractive surface 102 a again by the diffraction effect,whereby the light can be extracted outside the element in a plurality ofmodes.

In the LED element 101 having such a configuration, it is possible todecrease the thickness of the semiconductor lamination part 119 withoutdeteriorating the crystal quality of the light-emitting layer 114 bysetting the proportion of the flat part 102 b to 41% or more. Moreover,it is possible to improve the crystal quality of the light-emittinglayer 114 and to further improve the light extraction efficiency as longas the semiconductor lamination part 119 is as thick as the conventionalsemiconductor lamination part.

Moreover, since the verticalizing moth-eye surface 102 a is provided,light incident on the interface between the sapphire substrate 102 andthe group-III nitride semiconductor layer at an angle exceeding thecritical angle for total reflection can be directed closer to thevertical direction. Moreover, since the transmissive moth-eye surface127 g is provided, it is possible to suppress the Fresnel reflection oflight which is directed toward the vertical direction at the interfacebetween the sapphire substrate 102 and the outside of the element. Inthis way, the light extraction efficiency can be dramatically improved.

Further, the distance of light, emitted from the light-emitting layer114, until reaching the front surface of the p-side electrode 127, canbe reduced substantially, and the absorption of light in the element canbe suppressed. The LED element has such a problem that light is absorbedin the element as light in the angle region exceeding the critical angleof the interface propagates laterally. However, light in the angleregion exceeding the critical angle is directed toward the vertical atthe verticalized moth-eye surface 102 a, and thus the light absorbed inthe element can be reduced drastically.

Moreover, light can be extracted using a large number of diffractionmodes when the front surface of the sapphire substrate 102 has aplurality of projections 102 c disposed at the intersections of virtualtriangular lattices in a plan view thereof, and the triangles that formthe virtual triangular lattice do not have a regular polygonal shape. Inparticular, the light extraction efficiency can be improved when thelength of one side of triangles that form the virtual triangular latticeis twice or more than the optical wavelength of blue light and is 460 nmor smaller or between 550 nm and 800 nm. Moreover, when the trianglesthat form the virtual triangular lattice have an isosceles triangularshape, it is possible to increase the number of diffraction modes whileregularly disposing the projections 102 c. Further, light can beextracted using diffraction modes having different properties when thelength of equilateral sides of an isosceles triangle is twice or morethan the optical wavelength of blue light and is 400 nm or smaller andthe length of the bottom side is between 550 nm and 800 nm.

Here, the present inventors have found out that, by using thecombination of the dielectric multilayer film 124 and the metal layer126 as the reflection part at the back surface of the sapphire substrate102, the light extraction efficiency of the LED element 101 increasessubstantially. Namely, when the dielectric multilayer film 124 and themetal layer 126 are combined, the reflectivity increases as the anglecomes closer to the vertical with respect to the interface, whichattains favorable reflection condition for the light directed toward thevertical with respect to the interface.

FIG. 20 is a graph illustrating the reflectivity of a reflection partaccording to an example 3. According to the example 3, five pairs ofZrO₂ and SiO₂ are combined to form the dielectric multilayer film formedon the sapphire substrate, and the Al layer is formed to overlap thedielectric multilayer film. As illustrated in FIG. 20, the reflectivityof 99% or more is realized in the angle region where the angle ofincident is from 0 degree to 55 degrees. Furthermore, the reflectivityof 98% or more is realized in the angle region where the angle ofincident is from 0 degree to 60 degrees. Furthermore, the reflectivityof 92% or more is realized in the angle region where the angle ofincident is from 0 degree to 75 degrees. Thus, the combination of thedielectric multilayer film and the metal layer attains the favorablereflection condition for the light directed toward the vertical withrespect to the interface.

FIG. 21 is a graph illustrating the reflectivity of a reflection partaccording to an example 4. According to the example 4, only the Al layeris formed on the sapphire substrate. As illustrated in FIG. 21, thereflectivity shows 88% almost constantly, irrespective of the angle ofincident. Thus, the reflection part may be a single layer of metal, suchas the Al layer.

FIG. 22 is a schematic sectional view of an LED element according to athird embodiment of the present invention.

As illustrated in FIG. 22, an LED element 201 is a face-up type LEDelement in which a semiconductor lamination part 219 formed of agroup-III nitride semiconductor layer is formed on a front surface of asapphire substrate 202. The semiconductor lamination part 219 has abuffer layer 210, an n-type GaN layer 212, a light-emitting layer 214,an electron blocking layer 216, and a p-type GaN layer 218 in that orderfrom the side of the sapphire substrate 202. A p-side electrode 227 isformed on the p-type GaN layer 218, and an n-side electrode 228 isformed on the n-type GaN layer 212.

As illustrated in FIG. 22, the buffer layer 210 is formed on the frontsurface of the sapphire substrate 2 and is formed of MN. In the presentembodiment, the buffer layer 210 is formed by an MOCVD (Metal OrganicChemical Vapor Deposition) method, but may be formed by a sputteringmethod. The n-type GaN layer 212 as a first conductivity-type layer isformed on the buffer layer 210 and is formed of n-GaN. Thelight-emitting layer 214 is formed on the n-type GaN layer 212 and isformed of GaInN/GaN and emits blue light by electron and hole injection.Here, the blue light means light whose peak wavelength is between 430 nmand 480 nm, for example. In the present embodiment, a peak wavelength ofthe light emitted from the light-emitting layer 214 is 450 nm.

The electron blocking layer 216 is formed on the light-emitting layer214 and is formed of p-AlGaN. The p-type GaN layer 218 as a secondconductivity-type layer is formed on the electron blocking layer 216 andis formed of p-GaN. The layers ranging from the n-type GaN layer 212 tothe p-type GaN layer 218 are formed by epitaxial growth of the group-IIInitride semiconductor. Although projection parts 2 c are periodicallyformed on the front surface of the sapphire substrate 2, planarizationis realized by lateral growth in the initial stage of the growth of thegroup-III nitride semiconductor. Moreover, the layer configuration ofthe semiconductor layer is optional as long as the semiconductor layerincludes at least a first conductivity-type layer, an active layer, anda second conductivity-type layer, and the active layer emits light byelectron-hole recombination when a voltage is applied to the firstconductivity-type layer and the second conductivity-type layer.

The front surface of the sapphire substrate 202 forms a verticalizingmoth-eye surface 202 a, and the front surface of the p-side electrode227 forms a transmissive moth-eye surface 227 g. A flat part 202 b and aplurality of projection parts 202 c that are periodically formed on theflat part 202 b are formed on the front surface of the sapphiresubstrate 202. Moreover, the sapphire substrate 202 is configured sothat the proportion of the flat parts 202 b is 40% or more in a planview thereof. Examples of the shape of each projection part 202 cinclude a pyramidal shape such as a conical shape or a polygonalpyramidal shape and a truncated pyramidal shape obtained by cutting thetop of a pyramid such as a truncated conical shape or a truncatedpolygonal pyramidal shape. Each projection part 202 c is designed todiffract light emitted from the light-emitting layer 214. In the presentembodiment, the light verticalizing effect can be obtained by theprojection parts 202 c disposed periodically. Here, the lightverticalizing effect means an effect by which light intensitydistribution after the light is reflected from and transmitted throughthe verticalizing moth-eye surface is inclined closer to the directionvertical to the interface between the sapphire substrate 202 and thesemiconductor lamination part 219 than before the light is incident onthe verticalizing moth-eye surface.

A dielectric multilayer film 224 is formed on the back surface of thesapphire substrate 202. The dielectric multilayer film 224 is covered byan Al layer 226 which is a metal layer. In this light-emitting element201, the dielectric multilayer film 224 and the Al layer 226 form areflecting portion, and light emitted from the light-emitting layer 214and transmitted through the verticalizing moth-eye surface 202 a by thediffraction effect is reflected from the reflecting portion. The lighttransmitted by the diffraction effect is incident on the diffractivesurface 202 a again and is transmitted through the diffractive surface202 a again by the diffraction effect, whereby the light can beextracted outside the element in a plurality of modes.

The p-side electrode 227 includes a diffusion electrode layer 221 formedon the p-type GaN layer 218 and a moth-eye layer 222 formed on thediffusion electrode layer 221. Moreover, in the present embodiment, thep-side electrode 227 includes a pad electrode 223 that passes throughthe diffusion electrode layer 221 and the moth-eye layer 222 so as tomake contact with the p-type GaN layer 218. The diffusion electrodelayer 221 is formed on the p-type GaN layer 218 excluding a formationregion of the pad electrode 223 and is formed of a transparent material.Moreover, the moth-eye layer 222 is formed on the diffusion electrodelayer 221 excluding the formation region of the pad electrode 223 and isformed of a transparent material. The moth-eye layer 222 is formed of amaterial that has a smaller extinction coefficient than a material thatforms the diffusion electrode layer 221 and has approximately the sameindex of refraction as the material that forms the diffusion electrodelayer 221. Approximately the same index of refraction means that adifference between the indices of refraction of the diffusion electrodelayer 221 and the moth-eye layer 222 is within 20% of the index ofrefraction of the moth-eye layer 222. Moreover, the diffusion electrodelayer 221 is formed of a material that has a smaller sheet resistancethan the moth-eye layer 222 and is formed thinner than the moth-eyelayer 222. Further, the thickness of the diffusion electrode layer 221is smaller than the thickness of the moth-eye layer 222. Moreover, thepad electrode 223 is formed of a metal material such as Al, for example.Moreover, the pad electrode 223 is formed of a material that exhibitsstronger adhesion to the semiconductor lamination part 219 than adhesionto the diffusion electrode layer 221.

In the present embodiment, the diffusion electrode layer 221 is formedof ITO (Indium Tin Oxide) having a thickness of 100 nm and the moth-eyelayer 222 is formed of ZrO₂ having a thickness of 400 nm. For lighthaving a wavelength of 450 nm, the extinction coefficient of ITO is 0.04and the extinction coefficient of ZrO₂ is approximately 0. Moreover, forlight having a wavelength of 450 nm, the index of refraction of ITO is2.04 and the index of refraction of ZrO₂ is 2.24. The diffusionelectrode layer 221 may be formed of a material such as IZO (Indium ZincOxide), for example, and the moth-eye layer 222 may be formed of amaterial such as Nb₂O₅, for example.

A flat part 227 h and a plurality of projection parts 227 i that areperiodically formed on the flat part 227 h are formed on the frontsurface of the moth-eye layer 222. Examples of the shape of eachprojection part 227 i include a pyramidal shape such as a conical shapeor a polygonal pyramidal shape and a truncated pyramidal shape obtainedby cutting the top of a pyramid such as a truncated conical shape or atruncated polygonal pyramidal shape. The period of the projection parts227 i on the transmissive moth-eye surface is smaller than twice theoptical wavelength of the light-emitting layer 214. In the presentembodiment, the respective projection parts 227 i disposed periodicallysuppress Fresnel reflection at the interface with the outside.

The n-side electrode 228 is formed on the n-type GaN layer 212 exposedafter etching the p-type GaN layer 218 to form the n-type GaN layer 212.The n-side electrode 228 is formed on the n-side GaN layer 212 and isformed of a metal material such as Al, for example.

FIG. 23 is an explanatory view illustrating the traveling direction oflight in the element.

As illustrated in FIG. 23, light incident on the sapphire substrate 202at an angle exceeding the critical angle, among the light componentsemitted from the light-emitting layer 214, is transmitted through andreflected from the verticalizing moth-eye surface 202 a toward thevertical direction, as compared to the direction when it is incident onthe verticalizing moth-eye surface 202 a. That is, light that isreflected the verticalizing moth-eye surface 202 a is incident on thetransmissive moth-eye surface 227 g by changing its angle so as to bedirected closer to the vertical direction. Further, light that passesthrough the verticalizing moth-eye surface 202 a, whose angle is changedso as to be directed toward the vertical, is reflected from a reflectingportion that is made up of the dielectric multilayer film 224 and the Allayer 226, and thereafter, is incident on the verticalizing moth-eyesurface 202 a again. The angle of incidence at this time is closer tothe vertical than the previous angle of incident. As a result, lightincident on the transmissive moth-eye surface 227 g can be directedtoward the vertical.

Moreover, the transmissive moth-eye surfaces 227 g of the p-sideelectrode 227 are formed so as to be aligned at the intersections ofvirtual triangular lattices with a predetermined period so that thecenters of the respective projection parts 227 i are positioned at thevertices of regular triangles in a plan view thereof. The shortestperiod of the projection parts 227 i is smaller than the opticalwavelength of the light emitted from the light-emitting layer 214. Thatis, the Fresnel reflection is suppressed in the transmissive moth-eyesurface 227 g. In the present embodiment, the length of one side of theregular triangles that form the virtual triangular lattice is 300 nm.That is, the shortest period of the projection parts 227 i is 300 nm.Since the wavelength of the light emitted from the light-emitting layer214 is 450 nm and the index of refraction of ZrO₂ is 2.24, the opticalwavelength thereof is 200.9 nm. That is, the shortest period of thetransmissive moth-eye surfaces 227 g is smaller than twice the opticalwavelength of the light-emitting layer 214. When the period of themoth-eye surfaces is equal to or smaller than twice the opticalwavelength, the Fresnel reflection at the interface can be suppressed.It is possible to sufficiently obtain the effect of suppressing theFresnel reflection when the shortest period of the projection parts 227i is smaller than twice the optical wavelength. Moreover, it is possibleto obtain a larger effect of suppressing the Fresnel reflection when allperiods of the projection parts 227 i are smaller than twice the opticalwavelength. The effect of suppressing the Fresnel reflection increasesas the period of the transmissive moth-eye surface 227 g approaches onetimes from two times the optical wavelength. When the outside of themoth-eye layer 222 is resin or air, and when the period of thetransmissive moth-eye surfaces 227 g is equal to or smaller than 1.25times the optical wavelength, it is possible to obtain approximately thesame effect of suppressing the Fresnel reflection as when the period isone times or smaller than the optical wavelength.

Next, a method of forming the transmissive moth-eye surface 227 g of thep-side electrode 227 will be described with reference to FIG. 24 andFIG. 25. FIG. 24 is an explanatory view for describing the processes ofprocessing a moth-eye layer, in which (a) illustrates a state where afirst mask layer is formed on a transmissive moth-eye surface, (b)illustrates a state where a resist layer is formed on the first masklayer, (c) illustrates a state where the resist layer is selectivelyirradiated with an electron beam, (d) illustrates a state where theresist layer is developed and removed, and (e) illustrates a state wherea second mask layer is formed.

First, as illustrated in FIG. 24(a), a first mask layer 330 is formed onthe front surface of the moth-eye layer 222. The first mask layer 330 isformed of SiO₂, for example, and is formed by a sputtering method, avacuum deposition method, a CVD method, or the like. Although thethickness of the first mask layer 330 is optional, the thickness is 1.0μm, for example.

Next, as illustrated in FIG. 24(b), a resist layer 332 is formed on thefirst mask layer 330 of the moth-eye layer 222. The resist layer 332 isformed of an electron beam-sensitive material such as ZEP (product ofZEON Corporation) and is applied to the first mask layer 330. Althoughthe thickness of the resist layer 332 is optional, the thickness isbetween 100 nm and 2.0 μm, for example.

Next, as illustrated in FIG. 24(c), a stencil mask 334 is set so as tobe separated from the resist layer 332. The gap between the resist layer332 and the stencil mask 334 is between 1.0 μm and 100 μm. The stencilmask 334 is formed of a material such as diamond or SiC, for example,and may have an optional thickness (for example, between 500 nm and 100μm). The stencil mask 334 has openings 334 a through which electronbeams pass selectively.

After that, as illustrated in FIG. 24(c), the stencil mask 334 isirradiated with electron beams so that the resist layer 332 is exposedto the electron beams having passed through the openings 334 a of thestencil mask 334. Specifically, the pattern of the stencil mask 334 istransferred to the resist layer 332 using an electron beam dose of 10 to100 μC/cm², for example.

After irradiation of electron beams is completed, the resist layer 332is developed using a predetermined developing solution. As a result, asillustrated in FIG. 24(d), the portions irradiated with electron beamsare eluted to the developing solution, and the portions which are notirradiated with electron beams remain, whereby openings 332 a areformed. When ZEP (product of ZEON Corporation) is used as the resistlayer 332, amyl acetate, for example, can be used as the developingsolution.

Next, as illustrated in FIG. 24(e), a second mask layer 336 is formed onthe first mask layer 330 on which the resist layer 332 is patterned. Inthis way, the second mask layer 336 is patterned on the first mask layer330 using irradiation of electron beams. The second mask layer 336 isformed of Ni, for example, and is formed by a sputtering method, avacuum deposition method, a CVD method, or the like. Although thethickness of the second mask layer 336 is optional, the thickness is 20nm, for example.

FIG. 25 is an explanatory view for describing the processes ofprocessing the moth-eye layer, in which (a) illustrates a state wherethe resist layer is completely removed, (b) illustrates a state wherethe first mask layer is etched using the second mask layer as a mask,(c) illustrates a state where the second mask layer is removed, (d)illustrates a state where the transmissive moth-eye surface is etchedusing the first mask layer as a mask, and (e) illustrates a state wherethe first mask layer is removed.

As illustrated in FIG. 25(a), the resist layer 332 is removed using aremoval solution. For example, the resist layer 332 can be removed byimmersing the same in the removal solution and irradiating the same withultrasound waves for a predetermined period. Specifically, diethylketone, for example, can be used as the removal solution. In this way,the pattern of the second mask layer 336 obtained by inverting thepattern of the openings 334 a of the stencil mask 334 is formed on thefirst mask layer 330.

Next, as illustrated in FIG. 25(b), the first mask layer 330 issubjected to dry-etching using the second mask layer 336 as a mask. Inthis way, openings 330 a are formed in the first mask layer 330 and thepattern of the first mask layer 330 is formed. In this case, an etchinggas is used such that the moth-eye layer 222 and the first mask layer330 have higher durability than the second mask layer 336. For example,when the first mask layer 330 is SiO₂, the second mask layer 336 is Ni,and a fluorine-based gas such as SF₆ is used, since Ni has an etchingselectivity of approximately 100 in relation to SiO₂, the first masklayer 330 can be patterned accurately.

After that, as illustrated in FIG. 25(c), the second mask layer 336 onthe first mask layer 330 is removed. When the first mask layer 330 isSiO₂ and the second mask layer 336 is Ni, Ni can be removed by immersingthe same in a solution of hydrochloric acid and nitric acid mixed at theratio of approximately 1:1 and diluted with water or subjecting the sameto dry-etching using an argon gas.

Subsequently, as illustrated in FIG. 25(d), the moth-eye layer 222 issubjected to dry-etching using the first mask layer 330 as a mask. Inthis case, since the portions of the moth-eye layer 222 in which thefirst mask layer 330 is removed are exposed to an etching gas, theinverted pattern of the openings 334 a of the stencil mask 334 can betransferred to the moth-eye layer 222. In this case, since the firstmask layer 330 has higher durability to the etching gas than themoth-eye layer 222, the portions which are not covered by the first masklayer 330 can be selectively etched. The etching ends when the etchingdepth of the moth-eye layer 222 reaches a predetermined depth. Here, achlorine-based gas such as Cl₂ and fluorine-containing gas, for example,can be used as the etching gas. Since the fluorine-based gas cannot etchITO, when ITO is used as the diffusion electrode layer 221, thediffusion electrode layer 221 will not be processed exceeding themoth-eye layer 222. That is, even when the moth-eye layer 222 has aminimum thickness necessary for forming the projection-and-depressionstructure and the diffusion electrode layer 221 is exposed duringetching, the diffusion electrode layer 221 will not be etched.

After that, as illustrated in FIG. 25(e), the first mask layer 330remaining on the moth-eye layer 222 is removed using a predeterminedremoval solution. When SiO₂ is used as the first mask layer 330, rarehydrofluoric acid, for example, can be used as the removal solution. Inthis case, if the mask formed of SiO₂ is also formed in the formationregion of the pad electrode 223 of the p-type GaN layer 218, the maskcan be removed together. After that, the pad electrode 223 is formed onthe moth-eye layer 222. In this way, after the projection parts 227 iare formed on the front surface of the p-side electrode 227, thesapphire substrate is diced and divided into a plurality of LED elements201, whereby the LED element 201 is manufactured.

In the LED element 201 having such a configuration, the current flowingfrom the semiconductor lamination part 219 toward the p-side electrode227 is diffused in the diffusion electrode layer 221 and flows into thepad electrode 223. In this case, since the diffusion electrode layer 221has a low sheet resistance, the current can be diffused accurately.Since the pad electrode 223 and the diffusion electrode layer 221 are indirect contact with each other, current flows directly from thediffusion electrode layer 221 to the pad electrode 223 without via themoth-eye layer 222. Here, since the pad electrode 223 is formed of amaterial that exhibits stronger adhesion to the semiconductor laminationpart 219 than adhesion to the diffusion electrode layer 221, the padelectrode 223 is not easily separated from the semiconductor laminationpart 219 due to mechanical load or the like.

On the other hand, the light incident on the p-side electrode 227 isextracted outside after passing through the diffusion electrode layer221 and the moth-eye layer 2220. Here, since the diffusion electrodelayer 221 having a high extinction coefficient is thin and the moth-eyelayer 222 having a low extinction coefficient is thick, it is possibleto decrease the amount of light absorbed in the p-side electrode 227. Asa result, it is possible to improve the light extraction efficiency ofthe LED element 201. Moreover, since the diffusion electrode layer 221and the moth-eye layer 222 have approximately the same index ofrefraction, it is possible to suppress total reflections at theinterface between the two layers.

Moreover, since the verticalizing moth-eye surface 202 a is provided,light incident on the interface between the sapphire substrate 202 andthe group-III nitride semiconductor layer at an angle exceeding thecritical angle for total reflection can be directed closer to thedirection vertical to the interface. Moreover, since the transmissivemoth-eye surface 227 g that suppresses the Fresnel reflection isprovided, light which is directed closer to the vertical direction canbe extracted outside of the element at the interface between thesapphire substrate 202 and the outside of the element. As a result, thelight extraction efficiency can be dramatically improved.

Further, the distance travelled by the light emitted from thelight-emitting layer 214 until reaching the transmissive moth-eyesurface 227 g can be reduced remarkably, and the absorption of light inthe element can be suppressed. LED elements have such a problem thatlight is absorbed in the element since light in the angle regionexceeding the critical angle of the interface propagates in a lateraldirection. However, since light in the angle region exceeding thecritical angle is directed toward the vertical direction in theverticalizing moth-eye surface 202 a, and the Fresnel reflection of thelight being directed toward the vertical direction is suppressed in thetransmissive moth-eye surface 227 g, the light absorbed in the elementcan be reduced dramatically.

Moreover, it is possible to decrease the thickness of the semiconductorlamination part 219 without deteriorating the crystal quality of thelight-emitting layer 214 by setting the proportion of the flat part 202b to 41% or more. Moreover, it is possible to improve the crystalquality of the light-emitting layer 214 and to further improve the lightextraction efficiency as long as the semiconductor lamination part 219is as thick as the conventional semiconductor lamination part.

Moreover, light can be extracted using a large number of diffractionmodes when the front surface of the sapphire substrate 202 has aplurality of projection parts 202 c disposed at the intersections ofvirtual triangular lattices in a plan view thereof, and the trianglesthat form the virtual triangular lattice do not have a regular polygonalshape. In particular, the light extraction efficiency can be improvedwhen the length of one side of triangles that form the virtualtriangular lattice is twice or more than the optical wavelength of bluelight and is 460 nm or smaller or between 550 nm and 800 nm. Moreover,when the triangles that form the virtual triangular lattice have anisosceles triangular shape, it is possible to increase the number ofdiffraction modes while regularly disposing the projection parts 102 c.Further, light can be extracted using diffraction modes having differentproperties when the length of equilateral sides of an isosceles triangleis twice or more than the optical wavelength of blue light and is 460 nmor smaller and the length of the bottom side is between 550 nm and 800nm.

Here, the present inventors have found that the light extractionefficiency of the LED element 201 increased remarkably when thecombination of the dielectric multilayer film 224 and the metal layer226 was used as the reflecting portion on the back surface of thesapphire substrate 202. That is, when the combination of the dielectricmultilayer film 224 and the metal layer 226 is used, the reflectivityincreases as the angle comes closer to the vertical with respect to theinterface, which attains a favorable reflection condition for the lightthat is directed toward the vertical with respect to the interface.

According to the above-described embodiments, although a configurationin which the verticalizing moth-eye surface and the transmissivemoth-eye surface are formed by the projection parts formed periodicallyhas been illustrated, the moth-eye surfaces can naturally be configuredby the depression parts formed periodically. Moreover, although aconfiguration in which the transmissive moth-eye surface is formed inthe p-side electrode has been illustrated, the transmissive moth-eyesurface may be further formed in the n-side electrode. Moreover, theprojection parts or the depression parts may be formed so as to bealigned at the intersections of virtual square lattices, for example,instead of forming the same at the intersections of the triangularlattices.

While the embodiments of the present invention has been described, theembodiments described above do not limit the inventions disclosed in theclaims. It should also be noted that all combinations of the featuresexplained in the embodiments are not necessarily essential to the meansfor solving the problem in the invention.

INDUSTRIAL APPLICABILITY

The LED light-emitting element and the method of manufacturing the sameaccording to the present invention are industrially useful because theelement and the method can further improve the light extractionefficiency.

REFERENCE SIGNS LIST

-   1 LED element-   2 Sapphire substrate-   2 a Verticalizing moth-eye surface-   2 b Flat part-   2 c Projection part-   2 d Side surface-   2 e Bent portion-   2 f Top surface-   2 g Transmission moth-eye surface-   2 h Flat part-   2 i Projection part-   10 Buffer layer-   12 N-type GaN layer-   14 Light-emitting layer-   16 Electron blocking layer-   18 P-type GaN layer-   19 Semiconductor lamination part-   21 Diffusion electrode-   22 Dielectric multilayer film-   22 a First material-   22 b Second material-   22 c Via hole-   23 Metal electrode-   24 Diffusion electrode-   25 Dielectric multilayer film-   25 a Via hole-   26 Metal electrode-   27 P-side electrode-   28 N-side electrode-   30 Mask layer-   31 SiO₂ layer-   32 Ni layer-   40 Resist film-   41 Projection-and-depression structure-   42 Residual film-   43 Projection part-   50 Mold-   51 Projection-and-depression structure-   91 Plasma etching apparatus-   92 Substrate holding table-   93 Container-   94 Coil-   95 Power supply-   96 Quartz plate-   97 Cooling controller unit-   98 Plasma-   101 LED element-   102 Sapphire substrate-   102 a Verticalizing moth-eye surface-   110 Buffer layer-   112 N-type GaN layer-   114 Light-emitting layer-   116 Electron blocking layer-   118 P-type GaN layer-   119 Semiconductor lamination part-   122 Pad electrode-   124 Dielectric multilayer film-   124 a First material-   124 b Second material-   126 Al layer-   127 P-side electrode-   127 g Transmission moth-eye surface-   128 N-side electrode-   201 LED element-   202 Sapphire substrate-   202 a Verticalizing moth-eye surface-   202 b Flat part-   202 c Projection part-   210 Buffer layer-   212 N-side GaN layer-   214 Light-emitting layer-   216 Electron blocking layer-   218 P-type GaN layer-   219 Semiconductor lamination part-   221 Diffusion electrode layer-   222 Moth-eye layer-   223 Pad electrode-   224 Dielectric multilayer film-   226 Al layer-   227 P-side electrode-   227 g Transmission moth-eye surface-   227 h Flat part-   227 i Projection part-   228 N-side electrode-   330 First mask layer-   330 a Opening-   332 Resist layer-   332 a Opening-   334 Stencil mask-   334 a Opening-   336 Second mask layer

The invention claimed is:
 1. An LED element comprising: a semiconductorlamination part that includes a light-emitting layer; a diffractivesurface on which light emitted from the light-emitting layer is incidentand on which projection parts are formed with a period larger than anoptical wavelength of the light and smaller than a coherence length ofthe light and which reflects the incident light in a plurality of modesaccording to a Bragg diffraction condition and transmits the incidentlight in a plurality of modes according to the Bragg diffractioncondition; and a reflecting surface that reflects light refracted by thediffractive surface so that the reflected light is incident on thediffractive surface again, wherein the semiconductor lamination part isformed on the diffractive surface without any void around the projectionparts, a proportion of a flat part in the diffractive surface is 40% ormore in a plan view thereof, the light-emitting layer emits blue light,the projection parts are disposed at the intersections of virtualtriangular or rectangular lattices in a plan view thereof, and thetriangles or rectangles that form the virtual triangular or rectangularlattice do not have a regular polygonal shape and the length of eachside is larger than twice the optical wavelength of the blue light andsmaller than the coherence length.
 2. The LED element according to claim1, wherein the projection parts are disposed at the intersections ofvirtual triangular lattices, and the length of one side of trianglesthat form the virtual triangular lattice is twice or more than theoptical wavelength of the blue light and is 460 nm or smaller or between550 nm and 800 nm.
 3. The LED element according to claim 2, wherein thetriangles that form the virtual triangular lattice are isoscelestriangles.
 4. The LED element according to claim 3, wherein the lengthof one of equilateral sides and a bottom side of the isosceles triangleis twice or more than the optical wavelength of the blue light and is460 nm or smaller or between 550 nm and 800 nm.
 5. The LED elementaccording to claim 4, wherein a front surface of the sapphire substrateforms a verticalizing moth-eye surface in which the projection parts aredisposed with a period larger than twice the optical wavelength of theblue light and smaller than the coherence length, the verticalizingmoth-eye surface is configured to reflect and transmit light incident onthe verticalizing moth-eye surface from the semiconductor laminationpart side, in an angle range exceeding a critical angle, an intensitydistribution of the light extracted by reflection from the verticalizingmoth-eye surface at the semiconductor lamination part side is inclinedcloser to a direction vertical to an interface between the semiconductorlamination part and the sapphire substrate than an intensitydistribution of the light incident on the verticalizing moth-eye surfaceat the semiconductor lamination part side, in an angle range exceedingthe critical angle, an intensity distribution of the light extracted bytransmission from the verticalizing moth-eye surface at the sapphiresubstrate side is inclined closer to the direction vertical to theinterface than an intensity distribution of the light incident on theverticalizing moth-eye surface at the semiconductor lamination partside, the LED element includes a reflecting portion that reflects lighthaving transmitted through the verticalizing moth-eye surface, the LEDelement includes a transmissive moth-eye surface having depression partsor projection parts formed with a period smaller than twice the opticalwavelength of the light emitted from the light-emitting layer, and thelight of which the intensity distribution is adjusted so as to beinclined closer to the direction vertical to the interface by reflectionand transmission at the verticalizing moth-eye surface is extractedoutside the element in a state where the Fresnel reflection at thetransmissive moth-eye surface is suppressed.
 6. The LED elementaccording to claim 5, wherein reflectivity of the reflecting portionincreases as an angle comes closer to the direction vertical to theinterface.
 7. The LED element according to claim 6, further comprising:an electrode formed on the semiconductor lamination part, wherein theelectrode includes a diffusion electrode layer formed on thesemiconductor lamination part and a moth-eye layer which is formed onthe diffusion electrode layer and of which the front surface forms thetransmissive moth-eye surface having depression parts or projectionparts formed with a period smaller than twice the optical wavelength ofthe light emitted from the light-emitting layer, and the moth-eye layeris formed of a material which has a smaller extinction coefficient thana material that forms the diffusion electrode layer with respect to thelight emitted from the light-emitting layer and has approximately thesame index of refraction as the material that forms the diffusionelectrode layer.
 8. The LED element according to claim 7, wherein thediffusion electrode layer is formed of ITO, and the moth-eye layer isformed of ZrO₂.
 9. A method of manufacturing the LED element accordingto claim 1, forming a mask layer on a front surface of a sapphiresubstrate; forming a resist film on the mask layer; forming apredetermined pattern on the resist film; etching the mask layer usingthe resist film as a mask; etching the sapphire substrate using theetched mask layer as a mask to form the projection parts; and formingthe semiconductor lamination part on the etched front surface of thesapphire substrate.
 10. The method of manufacturing the LED elementaccording to claim 9, wherein the mask layer includes a metal mask and asize at a base end of the projection part is controlled according to athickness of the metal mask.
 11. An LED element comprising: a sapphiresubstrate; and a semiconductor lamination part including alight-emitting layer that is formed on a front surface of the sapphiresubstrate so as to emit blue light, wherein the front surface of thesapphire substrate has a plurality of depression parts or projectionparts disposed at the intersections of virtual triangular or rectangularlattices in a plan view thereof, and the triangles or rectangles thatform the virtual triangular or rectangular lattice do not have a regularpolygonal shape and the length of each side is larger than twice theoptical wavelength of the blue light and smaller than the coherencelength.
 12. The LED element according to claim 11, wherein thedepression parts or projection parts are disposed at the intersectionsof virtual triangular lattices, and the length of one side of trianglesthat form the virtual triangular lattice is twice or more than theoptical wavelength of the blue light and is 460 nm or smaller or between550 nm and 800 nm.
 13. The LED element according to claim 12, whereinthe triangles that form the virtual triangular lattice are isoscelestriangles.
 14. The LED element according to claim 13, wherein the lengthof one of equilateral sides and a bottom side of the isosceles triangleis twice or more than the optical wavelength of the blue light and is460 nm or smaller or between 550 nm and 800 nm.
 15. The LED elementaccording to claim 11, wherein a front surface of the sapphire substrateforms a verticalizing moth-eye surface in which the depression parts orprojection parts are disposed with a period larger than twice theoptical wavelength of the blue light and smaller than the coherencelength, the verticalizing moth-eye surface is configured to reflect andtransmit light incident on the verticalizing moth-eye surface from thesemiconductor lamination part side, in an angle range exceeding acritical angle, an intensity distribution of the light extracted byreflection from the verticalizing moth-eye surface at the semiconductorlamination part side is inclined closer to a direction vertical to aninterface between the semiconductor lamination part and the sapphiresubstrate than an intensity distribution of the light incident on theverticalizing moth-eye surface at the semiconductor lamination partside, in an angle range exceeding the critical angle, an intensitydistribution of the light extracted by transmission from theverticalizing moth-eye surface at the sapphire substrate side isinclined closer to the direction vertical to the interface than anintensity distribution of the light incident on the verticalizingmoth-eye surface at the semiconductor lamination part side, the LEDelement includes a reflecting portion that reflects light havingtransmitted through the verticalizing moth-eye surface, the LED elementincludes a transmissive moth-eye surface having depression parts orprojection parts formed with a period smaller than twice the opticalwavelength of the light emitted from the light-emitting layer, and thelight of which the intensity distribution is adjusted so as to beinclined closer to the direction vertical to the interface by reflectionand transmission at the verticalizing moth-eye surface is extractedoutside the element in a state where the Fresnel reflection at thetransmissive moth-eye surface is suppressed.
 16. The LED elementaccording to claim 15, wherein reflectivity of the reflecting portionincreases as an angle comes closer to the direction vertical to theinterface.